Screws having a head including an underhead bearing surface are generally known from the prior art, for example from German Patent Application No. DE 10 2008 041 391 A1, German Patent Application No. DE 100 01 857 A1 corresponding to U.S. patent application Ser. No. US 2003/0039527 A1 and German Patent Application No. DE 36 41 836 A1. None of these screws includes lubricant pockets.
Furthermore, it is known that screws include coatings for attaining defined friction between the underhead bearing surface and the component to be connected by screwing. These coatings are often furthermore used to improve corrosion protection.
If such screws are tightened for a plurality of times, i.e. they are untightened after tightening and then tightened again, this often leads to undesired galling effects. The applied coating is worn off, and there is metallic contact between the underhead bearing surface of the screw and the component to be connected by screwing. In this way, the friction acting in the contact surface is increased in a substantial and an uncontrollable way. Due to the great variation of the friction, secure multiple tightening processes are then no longer possible.
One especially observes this undesired phenomenon in case of coatings having comparatively low adhesion acting between the coating and the base metal of the screw, as it is the case with organic coatings on the basis of waxes, polymers and the like.